Fluid jet polishing (FJP) is a method of contouring and polishing a surface of a component by aiming a jet of a slurry of working fluid at the component and eroding the surface to create a desired shape. Fluid jet polishing has been studied in some detail, in particular by Silvia M. Booij see ISBM 90-9017012-X, 2003.
A conventional fluid jet polishing system 1, illustrated in FIGS. 1 and 2, comprises the following: a part holder 2, which holds a component 3 to be eroded; a contained area 4a with a drain 4b; a volume of working fluid 5, e.g. water, glycol, oil or other suitable fluids; a pump 6 to pressurize the working fluid 5; and plumbing 7 to return the working fluid 5 to a nozzle 8, which directs the working fluid 5 at the component 3. A motion system 10, usually computer controlled, directs the nozzle 8.
The profile of the effect of a stationary fluid jet on the surface of the component 3 creates a tool pattern. A computer program is then used to optimize the dwell time of the tool pattern on the surface of the component 3 in order to achieve the desired final surface figure. Typically the pressure of the slurry of working fluid 5 remains constant and the velocity (or dwell time) of the nozzle 8 is varied to remove the desired amount of material from different areas of the component 3. Alternatively the nozzle 8 can remain fixed and the component 3 can be moved. A temperature controller may be added to maintain the fluid at a constant temperature.
One of the key challenges with FJP is creating a uniform continuous stream of the working fluid 5. Typically, the working fluid 5 contains small abrasive particles made from hard materials, such as Aluminum Oxide, Diamond and/or Zirconium Oxide in a carrier fluid, e.g. water or similar fluid. The small abrasive particles have a certain negative buoyancy in the working fluid, whereby the impact of the abrasive particles on the surface of the component 3 depends on the speed of the abrasive particles and the buoyancy of the abrasive particles in the working fluid 5. However, air bubbles in the slurry can cause inconsistency in the polishing by dramatically altering the buoyancy of the particles, which causes the particles to damage the surface of the component 3 and increase the surface roughness of the finished surface. The viscosity of air is also much lower than the carrier fluid, so the movement of the abrasive particles to the interface between the working fluid 5 and the surface of the component 3 is faster.
When the jet of working fluid 5 impacts the surface of the component 3, the direction of the flow changes. As the working fluid 5 changes direction, particles suspended therein change direction and experience a force in the direction of the surface. The greater the density difference between the abrasive particles and the working fluid 5, the higher the force toward the surface will be. Centrifugal force drive the particles into the surface and creates the tool profile. The centrifugal force is resisted by the viscosity of the carrier fluid. Smaller abrasive particles have a larger ratio of cross sectional area to mass, which also decreases the ratio of centrifugal force to viscous drag. The response of materials tested to date with the fluid jet process indicates a non-linear response to increasing the centrifugal force/drag ratio. In a Newtonian fluid (viscosity constant with shear, for example water), an abrasive particle density of 7 g/cm3 or more is preferred.
Particle size not only affects the centrifugal force/drag ratio, but also affects the material removal rate. Larger abrasive particles increase the material removal rate, but also increase the finished surface roughness.
Another similar technology, disclosed in U.S. Pat. No. 5,951,369 issued Sep. 14, 1999 to Kordonski et al, is called Magneto Rheological Finishing, (MRF). The technology uses a liquid slurry that is directed to a wheel, where it is stiffened by magnetic fields. The stiff slurry is then carried by the wheel into contact with the component to be finished. After rubbing past the component and causing erosion the slurry is then returned to its liquid state for re-circulation by removal from the magnetic field. The advantage of MRF is that the stiffened slurry provides rapid material removal. The disadvantage is that the magnet and wheel technology makes the process significantly more complex and expensive than fluid jet polishing.
An object of the present invention is to overcome the shortcoming of the prior art by providing a relatively simple, but highly effective fluid jet polishing system providing much smoother and much more accurately figured surfaces than conventional polishing systems.